Segmented Gain-Doping Of An Optical Fiber

ABSTRACT

The present disclosure provides an approach to more efficiently amplify signals by matching either the gain materials or the pump profile with the signal profile for a higher-order mode (HOM) signal. By doing so, more efficient energy extraction is achieved.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication Ser. No. 60/888,114, filed 2007 Feb. 5, having the title“Higher Order Mode Amplifiers,” which is incorporated herein byreference in its entirety.

Additionally, the following U.S. patent applications are incorporated byreference herein, as if expressly set forth in their entireties:

(a) U.S. patent application Ser. No. 11/606,718, filed on 2006 Nov. 30,by DiGiovanni et al.;

(b) U.S. patent application Ser. No. 11/230,905, filed on 2005 Sep. 20,by Nicholson et al.;

(c) U.S. patent application Ser. No. 11/105,850, filed on 2005 Apr. 14,by Ramachandran et al.;

(d) U.S. patent application Ser. No. 11/367,495, filed on 2006 Mar. 4,by Ramachandran et al.;

(e) U.S. patent application Ser. No. 11/487,258, filed on 2006 Jul. 14,by Fini et al.

Also, the following U.S. patent applications, which are being filedconcurrently, are incorporated by reference herein, as if set forth intheir entireties:

(f) [Docket Number FENA 001363], by Ramachandran and Yablon, having thetitle “Preventing Dielectric Breakdown in Optical Fibers”;

(g) [Docket Number FENA 001364], by DiGiovanni and Ramachandran, havingthe title “Sequentially Increasing Effective Area in Higher-Order Mode(HOM) Signal Propagation”;

(h) [Docket Number FENA 001365], by Ramachandran, having the title“Pumping in a Higher-Order Mode that is Different From a Signal Mode”;

(i) [Docket Number FENA 001367], by DiGiovanni and Ramachandran, havingthe title “Selectively Pumping a Gain-Doped Region of a Higher-OrderMode Optical Fiber”; and

(j) [Docket Number FENA 001368], by DiGiovanni and Headly, having thetitle “Pumping in a Higher-Order Mode that is Substantially Identical toa Signal Mode.”

FIELD OF THE DISCLOSURE

The present disclosure relates generally to optical fibers and, moreparticularly, to higher-order mode (“HOM”) signal transmission inoptical fibers.

BACKGROUND

Ever since silica-based optical fibers have been used for high-powerlasers and amplifiers, there have been ongoing efforts to increase thepower of the signal that is transmitted through the fibers, and also toimprove energy efficiency during signal amplification.

In conventional laser pumping, energy is transferred from an externalsource to a gain-doped fiber (or other laser gain medium). Thattransferred energy is absorbed in the fiber, thereby producing excitedstates in the atoms within the fiber. At a given point, the number ofparticles in one excited state exceeds the number of particles in theground state (or another less-excited state). In this condition, knownas population inversion, stimulated emission can occur, and the fibercan act as a laser or an optical amplifier. Thus, when a signal isinjected into the fiber, the pump energy is transferred from the gainmedium to the injected signal, thereby amplifying the injected signal asit propagates along the fiber.

Efforts have been made to alter the profile for the injected signal.However, what has not been extensively studied is the effect of shapingthe pump.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1A is a schematic showing an example setup for converting both apump and a signal to a higher-order mode (HOM).

FIG. 1B is a schematic showing another example setup for converting botha pump and a signal to a HOM.

FIG. 2A is a schematic showing a third example setup for converting botha pump and a signal to a HOM.

FIG. 2B is a schematic showing a fourth example setup for convertingboth a pump and a signal to a HOM.

FIG. 3 is a chart showing an example index profile of an optical fiber.

FIG. 4A is a diagram showing a near-field image of a cross-section of anoptical fiber.

FIG. 4B is a diagram showing a near-field image of an example signalthat is transmitted along the fiber of FIG. 4A.

FIG. 5A is a chart showing an index profile of an optical fiber and acorresponding HOM signal that can be transmitted along the opticalfiber.

FIG. 5B is a chart showing a signal profile of a HOM signal and acorresponding fiber-gain-doping profile.

FIG. 6A is a diagram showing a near-field image of an example HOMsignal.

FIG. 6B is a diagram showing the pumped region that corresponds to theHOM signal of FIG. 6A.

FIG. 7A is a diagram showing a near-field image of an example HOMsignal.

FIG. 7B is a diagram showing the pumped region that corresponds to theHOM signal of FIG. 7A.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference is now made in detail to the description of the embodiments asillustrated in the drawings. While several embodiments are described inconnection with these drawings, there is no intent to limit thedisclosure to the embodiment or embodiments disclosed herein. On thecontrary, the intent is to cover all alternatives, modifications, andequivalents.

As noted above, in conventional laser pumping, energy is transferredfrom an external source to a gain-doped fiber, thereby producing excitedstates in the atoms within the fiber. When population inversion isachieved, stimulated emission can occur, and the fiber can act as alaser or an optical amplifier. Thus, when a signal is injected into thefiber, the pump energy is transferred from the gain medium to theinjected signal, thereby amplifying the injected signal as it propagatesalong the fiber.

Rare-earth-doped amplifiers (e.g., those doped by Erbium (Er) orYtterbium (Yb)), which produce high-power signals at around the1.5-micrometer wavelength, are often cladding pumped, meaning that thepump is introduced into the cladding, which is not gain-doped. The pumplight in the cladding, through various known reflective and refractivemechanisms, eventually enters the gain-doped region of the opticalfiber, thereby resulting in population inversion in the gain-dopedregion.

Unfortunately, for fibers that have a very small gain-dopedcross-sectional area as compared to the cladding region, which is notgain-doped, the pump absorption length becomes large. The increase inabsorption length results in lower excitation levels and, also, acompromised power efficiency.

To remedy this problem, it is desirable to pump the core (or othergain-doped region), rather than the cladding, which is typically notgain-doped. However, suitable pump diodes, which can directly coupleinto the gain-doped region, do not have sufficient power. In someembodiments of the invention, as shown in FIGS. 1 and 2 herein, acascaded Raman resonator (CRR) is used to pump the core (or othergain-doped region). It should be appreciated that the invention is notlimited to the use of CRR. Rather, other fiber lasers can be used topump the gain-doped region of the optical fiber.

Also shown in FIGS. 1A, 1B, 2A, and 2B are various approaches to furtherincrease the conversion efficiency by suitably tailoring the pump tomatch the spatial intensity profile of the signal that is beingamplified. By matching the spatial intensity profile of the pump to thespatial intensity profile of the signal, more effective energyextraction is achieved. The reason being that, when the pump energy isnot transferred to the signal, this results in amplified spontaneousemissions (ASE), which, as is known in the art, is an undesired effect.

Referring now to the diagrams, FIG. 1A shows an example architecture forconverting both a pump and a signal to a higher-order mode (HOM). Inparticular, FIG. 1A shows an embodiment using a cascaded Raman resonator(CRR) 115 as the pump source. The pump is multiplexed with the signal110 at an input fiber 160 and the multiplexed light is launched into again-doped fiber 155, which is spliced 120 to the input fiber 160. Sincethe CRR 115 pumps the gain-doped region at the launch end, rather thanpumping the outer cladding (which is not gain-doped), this configurationincreases power efficiency, compared to conventional cladding pumping,by confining the pump light to the gain-doped area where the energyconversion occurs.

In FIG. 1A, a mode converter 135, such as a long-period grating (LPG),is placed some distance along the gain fiber 155, allowing the signal tobe amplified in the fundamental LP₀₁ mode 125 before conversion to ahigher-order mode (HOM) signal. The gain fiber 155 has a profile similarto that shown in FIG. 3. Since one having ordinary skill in the art isfamiliar with these index profiles, FIG. 3 is not discussed in greatdetail herein. Suffice it to say that, in the embodiment of FIG. 1A, thecore (d_(core)) and the inner cladding (d_(iclad)) are both gain-dopedso that signals traveling through core and the inner cladding will beamplified.

Continuing with FIG. 1A, the mode converter 135 can be designed to havemultiple peaks in conversion efficiency, thereby accommodating both thepump wavelength and the signal wavelength. Due to the high extinction ofmode converters, light that is not resonant with the mode converter 135will pass through the converter 135 with little attenuation ordistortion.

While peaks are shown for wavelengths of 1480 and a range between1500-1600 (shown here as 15xx)), it should be appreciated that the modeconverter 135 can be designed to accommodate virtually any wavelengthcombinations. Since mode converters can be designed and fabricated withwideband operation, especially when operating at the turn-around point(TAP), if the pump and the signal wavelengths are within the conversionbandwidth, then both the signal and the pump will be converted to thesame HOM as the light 145 continues to propagate down the gain-fiber155.

FIGS. 7A and 7B are diagrams showing near-field images of an examplesignal 610 and pump 720, respectively, which have both been converted toa HOM such that the intensity profile of the pump 720 substantiallyoverlaps with the intensity profile of the signal 610. While FIGS. 7Aand 7B show the HOM being the LP₀₆ mode, it should be appreciated thatthe pump and signal can be converted to other HOMs as desired or needed.Since, as shown in FIGS. 7A and 7B, the LP₀₆ pump 720 now overlaps withthe LP₀₆ signal 610, the pump energy is more efficiently converted andthere is less likelihood of ASE from the “dark” regions.

In other embodiments, multiple converters can be used to separatelyconvert the pump and the signal. Also, for other embodiments, the signaland the pump need not be converted to the same HOM. Rather, the signalmay be converted to one HOM while the pump is converted to another HOM.FIG. 2A shows one embodiment in which the signal and the pump areconverted to different HOMs, using two separate, serially-placed modeconverters.

The architecture, as shown in FIG. 2A, comprises a signal source 205that launches the signal 210 and a CRR 215 for the pump. The signal 210and the pump are multiplexed onto an input fiber 255, which is spliced220 to a gain-doped fiber 250. Similar to the embodiment described withreference to FIG. 1A, the gain-doped fiber of FIG. 2A has a gain-dopedcore and a gain-doped inner cladding.

The multiplexed LP₀₁ light 225 travels some distance along the fiber250, thereby allowing the signal to be amplified in the fundamental LP₀₁mode. At some point, a signal mode converter converts the signal toLP_(0m) mode 235, while passing the pump with little-to-no distortion orattenuation. FIGS. 6A and 6B show a specific example where m=6. Uponpassing through the signal mode converter 230, the resulting LP₀₆ signalwill appear similar to that shown in FIG. 6A while the pump, whichpasses through the signal mode converter 230 without conversion, appearssimilar to that shown in FIG. 6B.

Subsequent to passing through the signal mode converter 230, the HOMsignal 610 and the pump 620 pass through a pump mode converter 240,which is specifically configured to affect only the pump wavelength. Assuch, the HOM signal 610 passes through the pump mode converter withlittle-to-no distortion or attenuation, while the LP₀₁ pump getsconverted to a HOM. The converted light 245, which now includes the HOMsignal and the HOM pump, continues to propagate down the gain fiber 250.

While the signal mode converter 230 and the pump mode converter 240 areshown in FIG. 2A as being located serially along the gain fiber 250, itshould be appreciated that, if the mode converters are constructed ofLPGs or other comparable structures, the mode converters 230, 240 may bewritten at the same physical location in the gain fiber 250.Alternatively, the serial order of the mode converters can be rearrangedwithout detrimental effect on the operation of the apparatus. Also, asshown in the embodiment of FIG. 2A, the HOM of the pump (n) need not bethe same as the HOM of the signal (m).

Reasons exist for converting the signal and the pump to different HOMs(i.e., m≠n). For example, the pump mode converter 240 could be awide-band converter constructed near the grating TAP to accommodateuncertainty or drift in pump wavelength or wavelength-multiplexing ofseveral pumps. The signal mode converter 230 may be a narrow-band deviceto filter unwanted wavelength components. For this particularcircumstance, it may be desirable to have the pump mode (n) be differentfrom the signal mode (m).

Returning to the embodiment of FIG. 1A, to improve mode extinction,unwanted modes from splices and scattering can be stripped bystrategically placing an optional HOM mode stripper 130 prior to themode converter 135. Another embodiment without the HOM mode stripper 130is shown as FIG. 1B. Since the architecture of FIG. 1A is substantiallysimilar to the architecture of FIG. 1B, with the exception of the modestripper 130, a description of any duplicative items is omitted herein.

Continuing, the mode stripper 130, which is shown in FIG. 1A, can befabricated as a short length of fiber, over which the coating is removedand the fiber is tapered or etched to cause light in the cladding to bestripped away. As such, only the light that is guided by the centralcore (which includes both the pump and signal) will be retained. Inother words, the mode stripper 130 removes (or strips away) variousundesired HOMs that may be present in the system, and the mode stripper130 in combination with the mode converter 135 fulfills all filteringfunctions except that of in-band ASE in the same mode.

For example, light may reside in any number of HOMs that propagatebackward (or counter to the signal). These unwanted HOMs can result fromASE, or stimulated Brllouin scattering (SBS) arising in the HOM gainfiber 155, or reflected light. Such backward propagating light will notbe resonant with the mode converter 135 and will remain in the cladding,to be removed by the mode stripper 130.

Forward-propagating HOM light may originate at splices or fromscattering in or before the segment on the signal-side of the modestripper 130. The mode stripper 130 will also remove thisforward-propagating HOM light.

Additionally, any ASE generated in the desired HOM, but out-of-band withthe mode converter 135 will also not convert to the fundamental mode. Assuch, this out-of-band HOM will also be removed.

Also, backward-propagating in-band ASE that is generated in the LP₀₁fundamental mode of the HOM section will convert to LP₀₇ and will alsobe removed by the mode stripper 130.

In short, the mode stripper 130 will remove all but the in-band HOM andthe out-of-band fundamental mode. The in-band ASE, however, is typicallynot a problem. Also, the out-of-band fundamental mode may not beproblematic because the pump profile will, by definition, match that ofthe signal. As such, much of the problematic portions of the light willbe removed by the mode stripper 130, and any light that remainsunaffected by the mode stripper 130 will likely be benign.

Given the functionality of the mode stripper 130 in FIG. 1A, similarimprovements can be achieved in the embodiment of FIG. 2A bystrategically placing an optional HOM mode stripper 130 prior to themode converter 230. Such an embodiment, which includes the HOM modestripper 130, is shown in FIG. 2B. Since the remaining architecture ofFIG. 2B is substantially similar to the architecture of FIG. 2A, furtherdiscussion of other similar components in FIG. 2B is omitted here.

Since the CRR 115, 215 pumps the core (or other smaller-diametergain-doped region), it is worthwhile to examine the structure of thegain-doped fiber in greater detail. FIG. 4A is a diagram showing anear-field image of a cross-section of an optical fiber, and FIG. 4B isa diagram showing a near-field image of an example signal that istransmitted along the fiber of FIG. 4A.

As background, conventional double-clad amplifiers, including HOMamplifiers, are cladding pumped. These cladding-pumped amplifiersinclude a region that guides the signal (typically a core, which isgain-doped) and a region that guides the pump light (typically acladding, which is not gain-doped). The cross-sectional area of thecladding is normally much greater than the cross-sectional area of thecore. This difference in area accommodates low-brightness pumps that areused for high-power operation, which means that the rate of absorptionof pump light is reduced by an amount that is roughly proportional tothe ratio of the two areas. This results in a corresponding increase inthe fiber length.

Unfortunately, for high power amplifiers, this increase in lengthconcomitantly increases the amount of undesired length-dependentnonlinear effects. For this reason, it is desirable to guide the pump inthe same spatial region as the signal to increase the overlap.

In conventional large-mode area (LMA) fibers with low-brightness pumps,this is typically not possible because the core has a relatively lownumerical aperture (NA) to reduce the number of guided modes andmaintain a large mode area. In such designs, the pump guide (e.g.,cladding) has different characteristics, and therefore must be in adifferent spatial region that the signal guide (e.g., core).

For HOM propagation, this need not be so. Because the mode is robustlyguided and resistant to mode coupling, and because it can be excitedwith high extinction, the NA of the guide can be large. In fact, thepump guide and the HOM guide can be in the same region. Turning to thedrawings, FIG. 4A shows a cross-sectional image 410 of a fiber (orwaveguide) that can propagate HOM signals. The fiber comprises a centralcore (not visible in the image 410) and an inner cladding 430 thatsurrounds the core. Radially-exterior to the inner cladding 430 is aring of air holes 440, which, in turn, is surrounded by a silica ring450. While not shown, a polymer layer surrounds the entire structure.The refractive index profile of such a fiber is shown in FIG. 3.

The inner cladding 430 is supported by a thin silica web, which definesthe air holes. The thickness of the web is small enough to effectivelyconfine the light to the inner cladding 430 and prevent leakage of lightto the silica ring 450. Due to the large contrast in index between airand silica (shown as ΔN_(dd) in FIG. 3), the inner cladding 430 is ahigh NA guide (approximately 0.6 to 0.8), suitable for containing pumplight.

Moreover, since the NA can be significantly higher than conventionaldouble-clad fiber, which is constructed of low-index polymer rather thanair holes, the light-carrying capacity of the fiber in FIG. 4A isequivalent to a fiber that is about 50% larger in diameter. For some HOMdesigns, the pump guide can be pumped using free-space optics.

FIG. 4B shows a near-field image 420 of an example HOM signal that istransmitted along the fiber of FIG. 4A. In particular, the scale of FIG.4B is matched to the scale of FIG. 4A to show the correspondence betweenthe diameter of the outer-most ring on the LP₀₆ HOM signal and thediameter of the inner cladding 430. Given this correspondence, one cansee that the inner cladding 430, when gain-doped, is suitable forguiding HOMs. As such, the pump and the signal can have high spatialoverlap and be guided by the same gain-doped region 430 (core and innercladding). In other words, the same waveguide confines both the signaland pump, even though the signal and pump may reside in differentspatial modes.

Continuing with FIG. 4B, the intensity pattern on the HOM image 420exhibits slight modulation due to the shape of the perimeter of theinner cladding 430. In conventional double-clad fibers, significanteffort is expended to create a noncircular pump waveguide or induce modedistortion within the pump waveguide to more effectively couple pumplight to the smaller gain (or core) region. Unlike conventionaldouble-clad fibers, in FIG. 4A, the pump and signal are co-located, and,hence, the pump waveguide can be circular and undistorted.

For other embodiments, noncircular inner claddings can be configured.For example, rectangular inner claddings can be used, due to theirimproved heat-transfer characteristics over thin dimensions.

It is worthwhile to note that, while air-clad fibers are specificallyshown in FIG. 4A, the core-pumping concept can be extended topolymer-clad fibers and glass-clad fibers. For some embodiments thathave a very deep trench (ΔN_(dd) of FIG. 3 is very large), the trencharea (d_(dd) of FIG. 3) can be enlarged. As a consequence, the outerregion (d_(oclad) of FIG. 3) can be eliminated altogether.

As noted above, the embodiments of FIGS. 1 and 2 show various approachesto matching the intensity profile of the signal with the intensityprofile of the pump. By matching the intensity profiles, ASE from the“dark” regions can be reduced. Another alternative to reducing the ASEin these so-called “dark” regions is by selectively doping thegain-doped fiber so that the fiber regions that correspond to the “dark”regions of the signal will have no gain dopant. This is described ingreater detail with reference to FIGS. 5A and 5B.

FIG. 5A is a chart 500 showing an index profile 510 of an optical fiberand a corresponding HOM signal 520 that can be transmitted along theoptical fiber. As shown in FIG. 5A, an LP₀₈ HOM signal 520 is carriedalong an inner cladding, which, in this particular embodiment, extendsradially outward to approximately 40 micrometers. The “dark” regionscorrespond to the zero intensities on the HOM signal 520 plot.

FIG. 5B is a chart showing the HOM signal 520 of FIG. 5A and acorresponding fiber-gain-doping profile 560. As one can see, the fiberis doped with rare-earth (RE) dopants at distinct radial locations.Specifically, the location of the gain-dopants corresponds to eachintensity peak of the LP₀₈ HOM signal 520. Thus, unlike conventionalgain-doped fibers, which largely have a uniform distribution ofRE-dopants within the entire inner cladding, the embodiment of FIG. 5Bshows segmented doping of the fiber to correspond with the particularHOM signal that will be transmitted along that particular fiber.

One disadvantage of segmented doping, as compared to simply convertingthe pump to a HOM, is a reduced flexibility in mode selection. Once afiber has been gain-doped in specific regions, the intensity profile ofthe signal cannot be changed without destroying the signal's spatialcorrespondence with the gain-doped fiber profile. Additionally, anyanomalous doping can result in imperfect overlap of the gain-dopedregion and the HOM signal. For this reason, it may be preferable toemploy the approaches taught with reference to FIGS. 1 and 2, ratherthan the segmented-doping approach of FIGS. 5A and 5B.

Although exemplary embodiments have been shown and described, it will beclear to those of ordinary skill in the art that a number of changes,modifications, or alterations to the disclosure as described may bemade. For example, while specific HOMs have been shown in the drawingsand described in detail, it should be appreciated that other mode orders(in addition to those that are expressly shown) can be used toaccommodate various other design parameters. Additionally, whilespecific examples of doping profiles have been shown and described, itshould be appreciated that these specific doping profiles may be alteredto correspond, in varying degrees, to different HOM signals. All suchchanges, modifications, and alterations should therefore be seen aswithin the scope of the disclosure.

1. An optical fiber, comprising: a core having a first index ofrefraction; an inner cladding located radially-exterior to the core, theinner cladding having a second index of refraction, the second index ofrefraction being less than the first index of refraction; gain-dopantsdoping the inner cladding at predefined locations, the predefinedlocations corresponding to an intensity profile of a higher-order mode(HOM) signal; and a trench located radially-exterior to the innercladding, the trench having a third index of refraction, the third indexof refraction being less than the second index of refraction.
 2. Theoptical fiber of claim 1, the core being gain-doped.
 3. The opticalfiber of claim 1, the gain-dopants comprising rare-earth dopants.
 4. Theoptical fiber of claim 3, the rare-earth dopants comprising Erbium. 5.The optical fiber of claim 3, the rare-earth dopants comprisingYtterbium.
 6. An optical fiber, comprising: a core; an inner claddinglocated radially-exterior to the core; and gain-dopants doping the innercladding at predefined locations, the predefined locations correspondingto an intensity profile of a higher-order mode (HOM) signal.
 7. Theoptical fiber of claim 6, the inner cladding being a waveguide having ahigh numerical aperture.
 8. The optical fiber of claim 6, furthercomprising: a trench located radially-exterior to the inner cladding,the trench having an index of refraction that is less than the index ofrefraction of the inner cladding.
 9. The optical fiber of claim 8, thetrench comprising air.
 10. The optical fiber of claim 8, the trenchcomprising a silica web.
 11. The optical fiber of claim 8, the trenchcomprising a low index polymer.
 12. The optical fiber of claim 6, theintensity profile comprising concentric rings.
 13. The optical fiber ofclaim 6, the gain-dopants comprising rare-earth dopants.
 14. The opticalfiber of claim 7, the rare-earth dopants comprising Erbium.
 15. Theoptical fiber of claim 7, the rare-earth dopants comprising Ytterbium.